1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer program product.
2. Description of the Related Art
Image processing apparatuses that digitalize image data acquired with a scanner by scanning an original are known in the art. Some of such image processing apparatuses include a halftone detection unit that extracts halftone detection area, which can have various sizes, from the digital image data and then decide whether a target pixel, which is a target of processing, is inside any of those halftone areas. Such an image processing apparatus has been disclosed in, for example, Japanese Patent Application Laid-open No. 2003-46772. Because such image processing apparatuses employ a plurality of pixels of various sizes, they can detect halftone areas containing various number of lines ranging from low number of screen lines of about 65 lines per inch to high number of screen lines of about 200 lines per inch. Especially, for halftone dots that contain low number of screen lines; i.e., 100 or less lines per inch, a cycle of pixels included therein is larger, so that a halftone detection filter that is larger in size than that needed for detecting halftone dots that contain high number of screen lines. Thus, the image processing apparatus disclosed in Japanese Patent Application Laid-open No. 2003-46772 is suitable for detecting halftone dots that contain only low number of screen lines.
Complicated small characters, such as kanji characters, in digital image data can have areas where density-difference pattern are partially similar to that of a reference halftone pattern. Pixels in such an area are often erroneously detected as belonging to a halftone dot. Generally, not all the pixels that are detected as belonging to a halftone dot belong to a true halftone area. Pixels belonging to a halftone dot in a true halftone area are characterized in that they are present in high density and are lined up in a regular pattern. On the other hand, the pixels in the areas where density-difference pattern are partially similar to that of a reference halftone pattern are relatively scattered and are not lined up in a regular pattern. However, if a situation occurs where the pixels in the areas where density-difference pattern are partially similar to that of a reference halftone pattern show the same characteristics as the pixels belonging to a halftone dot in a true halftone area, it can lead to erroneous determination.